An elevator nowadays employs an induction motor as a motor, and in many cases the induction motor is driven by an inverter which can produce variable voltage and variable frequency (VVVF). In an elevator drive apparatus including such an induction motor and an inverter in combination, the speed control of the induction motor is generally an open-loop control by an voltage inverter for low speed elevators while for medium and high speed elevators a speed feedback control with a speed detection device is utilized.
In the open-loop speed control, the acceleration, constant speed, and deceleration corresponding to a speed pattern are realized by controlling the output frequency of the inverter and further the output voltage thereof based on the speed pattern.
The conventional open-loop speed control has an advantage that the speed detector is not required, resulting in low cost and no need for back-up means for a speed detector failure. However, since there is not speed detection means for motor speed, i.e., passenger cage speed, and for hoisting distance, precision in stopping is likely deteriorated by load fluctuation.